SUMMARY The disease outcome and pathology of tuberculosis (TB) are driven by the type of immune response mounted in the host, yet the molecular requirements for successful control of TB by immune cells remain poorly understood. In particular, although IL-10 production is upregulated in patients with active TB and is known to antagonize pathways that are essential for the control of Mycobacterium tuberculosis (Mtb) infection, the impact and function of IL-10 during Mtb infection has remained elusive because loss of function studies (i.e, in Il10-/- mice) have yielded only minimal phenotypes. We previously showed that the transcription factor Bhlhe40 regulates IL-10 expression in T cells in a mouse model of multiple sclerosis. By analyzing several published gene expression datasets, we have now found that BHLHE40 transcripts are present in significantly lower abundance in patients with active TB as compared to healthy controls and patients with latent TB. This finding of decreased BHLHE40 expression in patients with active TB led us to investigate its role during Mtb infection in mice. We discovered that loss of Bhlhe40 in mice during Mtb infection results in higher Il10 expression, higher bacterial burden, and early susceptibility to infection. The severe phenotypes observed in Bhlhe40-/- mice are similar to those observed in mice lacking STAT1 or NF-?B p50, both of which are transcription factors central to immune regulation. However, unlike STAT1 and NF-?B, a role for Bhlhe40 during infection is completely unknown. Through a series of detailed mechanistic studies, we find that Bhlhe40 is required to directly repress Il10 expression in T cells and CD11c+ cells during Mtb infection, and deletion of Il10 in Bhlhe40-/- mice reverses the susceptibility of these mice. Our preliminary data have led to our central hypothesis that Bhlhe40 functions as a key transcriptional regulator of gene expression during Mtb infection that is required for effective control of Mtb replication. We will begin to address this hypothesis by dissecting the mechanism by which Bhlhe40 regulates IL-10 production and how this impacts susceptibility to Mtb. Our studies are the first to investigate a role for Bhlhe40 in both infectious disease and in myeloid cells, and will provide fundamental insights into the molecular requirements for immunity to infection. In addition, investigations into Bhlhe40 provide a unique opportunity to shed light on how different levels of IL-10 can impact TB disease, something that has not been evident in other studies. To test our hypothesis and improve our understanding in these areas, we will address the following independent aims: (1) Identify how loss of Bhlhe40 in T cells and CD11c+ cells impacts on immune responses to Mtb, (2) Dissect the mechanism whereby Bhlhe40 regulates Il10 expression in immune cells, and (3) Define the immune responses that are responsible for susceptibility to Mtb infection in the presence of higher Il10 expression.